This invention relates to the manufacture of electrochromic devices for modulation of light transmission, absorption and reflection.
Electrochromic materials are materials whose optical properties can be reversibly altered in response to an applied potential in a process involving the simultaneous insertion or extraction of electrons and charge compensating ions. These materials have been used, e.g., in display devices, variable reflectance mirrors, and in windows for controlling light transmission.
In general, devices having a composite whose transmittance can be varied in response to an applied electrical potential are known. The composite includes a variably transmissive electrochromic layer that is normally colorless, but when reduced by the insertion of an electron and charge compensating ion becomes colored by absorption, reflectance or a combination of both. The composite also includes a second electrochromic layer, a so-called counter electrode, which is either optically passive when oxidized or reduced, or is colored when oxidized and colorless when reduced, thus forming a complement with the first electrochromic layer. The oxidation and reduction of the counter electrode must also occur by electron injection and insertion of the same charge compensating ion as the first electrochromic layer. The charge compensating ions are transported by an ion conducting but electron blocking layer, e.g. an electrolyte, separating the two electrodes.
A general equation representing the operation of the device in which WO.sub.3 is the primary electrochromic layer, V.sub.2 O.sub.5 the counter electrode, and Li.sup.+ the transported ion can be written as: ##STR1##
Here, b denotes that the two electrode materials need not be present in any special molar ratio. The degree to which the counter electrode V.sub.2 O.sub.5 is reduced in the bleached state is given by the stoichiometric parameter x. Coloration is accomplished by the transfer of Li from V.sub.2 O.sub.5 to WO.sub.3. Thus, assuming that V.sub.2 O.sub.5 is the limiting electrode and that therefore sufficient WO.sub.3 is present to accept all of the available Li, the dynamic optical range of the device will be determined by the amount of Li present.
The inserting species, Li, must be introduced during fabrication. This can be accomplished by several methods, including direct vapor deposition of Li.sub.x V.sub.2 O.sub.5 (or other counter electrode), its reduction by elemental Li in a separate step, or electrochemical reduction of the counter electrode layer in an electrolytic solution of Li.sup.+. Alternatively, the Li may be introduced initially into the primary electrochromic layer (WO.sub.3 in the above equation), using similar methods; or the Li may be distributed between both electrodes by the same means, the resulting as-fabricated structure being in some intermediate state of coloration. In a large scale process vapor deposition is preferred, so that the multiple layers of the structure can be deposited on an in-line, continuous basis, as in a series of box coaters. In coating evenly large area glass or plastic for architectural or vehicular applications, sputtering is presently the industry standard.
In order to fabricate the device structure in FIG. 1 by vacuum processing such as by sputtering, the layers must be deposited in sequence, one on top of the other. For high sputter rates consistent with economical processing of large surface areas, sources with a high electrical conductivity are preferred. Since the component layers are oxides, they must be deposited in an oxidizing atmosphere, such as in pure O.sub.2, Ar/O.sub.2 or O.sub.2 /H.sub.2 O--i.e., they must be reactively sputtered from targets of the parent metals. The Li (or other insertion atom) must be deposited in a reducing or inert atmosphere, however. The resulting compounds, Li.sub.x V.sub.2 O.sub.5 or Li.sub.x WO.sub.3 for example, are easily oxidized. In fabricating an ion conducting layer on top of the reduced electrochromic layer, the Li.sup.+ conducting glass Li.sub.4 SiO.sub.4 or LiAISiO.sub.4 for example, it is necessary to expose it to an oxidizing atmosphere. There are several examples of prior art in which WO.sub.3 is converted to Li.sub.x WO.sub.3 by vacuum deposition, then capped with an ion conductor. Yoshimura et al. (Japanese Journal of Applied Physics 22, 152 (1983)) reported spontaneous coloration when WO.sub.3 /Li.sub.2 WO.sub.4 bilayers were allowed to sit in contact for a day, the intensity of coloration being proportional to the Li.sub.2 WO.sub.3 thickness. Oi (Applied Physics Letters 37, 244 (1980)) demonstrated the evaporation of Li.sub.3 N onto WO.sub.3, which gave rise initially to dry injection of the Li to form Li.sub.x WO.sub.3 (evolving N.sub.2), but eventually the Li.sub.3 N ion conductor built up on the WO.sub.3. Optical switching was further demonstrated in an ITO:Li.sub.x WO.sub.3 :Li.sub.3 N:Al structure polarized between.+-.6V. Problems with this approach include the red coloration of Li.sub.3 N, the fact that it cannot be conveniently sputtered, and that it would be easily oxidized on deposition of the contiguous counter electrode and conductive oxide layers. Haas, Goldner et al. (Proc. SPIE 823, 81 (1987)) reported deposition of the ion conductor LiNbO.sub.3, onto WO.sub.3, which resulted in spontaneous lithiation of the WO.sub.3. Optical 01 switching devices were configured and demonstrated using V.sub.2 O.sub.5 or In.sub.2 O.sub.3 as the counter electrode. A problem with this approach is that LiNbO.sub.3 has an electrical conductivity that is too high, particularly in the Li-depleted state, resulting in parasitic dc current flow during device operation.
It is therefore an object of the present invention to provide a process for reducing the electrochromic layer, then "capping" it with an ion conducting oxide without oxidizing the former material. The subsequent layers may then be deposited onto this "capped" layer without oxidizing significantly the reduced electrochromic underlayer.
It is a further object of this invention to achieve this deposition sequence using metallic sources, for high sputtering rates, and using targets that can be handled reasonably in the open air (i.e. are not pure alkali metals).